This invention relates in general to pressure vessels and more particularly to a layered head for a pressure vessel and to a method of producing such a head.
The typical pressure vessel has a cylindrical shell and heads that close the two ends of the shell. The shell and heads are manufactured as separate components and are thereafter joined together by welding to produce a unitary structure. The heads are somewhat dome-shaped to enable them to better withstand elevated pressures.
It is generally recognized that vessel walls composed of multiple layers are superior to thick single walls in many respects. For example, the individual plates of a layered wall, generally speaking, have better metallurgical properties than thick solid walls, since they are subjected to greater rolling at the mill. As a consequence, a layered vessel is usually safer than a solid wall vessel of equivalent wall thickness. Whereas thick solid walls exhibit a tendency to laminate, layered walls rarely display this tendency because of their better metallurgical properties. Also, in layered vessels it is possible to vary the metal alloy from layer to layer, thus enabling an expensive corrosion resistant liner to be used with less expensive surrounding layers. While thick steel plates clad with various corrosion resistant alloys are available from steel mills, they are more expensive. Moreover, thin layers are relatively easier to shape than the heavy steel plate used in solid wall vessels. Thus, layered walls can be manufactured in greater thickness than solid walls. Aside from that, the individual layers that comprise the wall of a layered vessel, upon being welded together, tend to shrink as the welds which join them solidify and cool, and this places the inner layers in a state of precompression. This is desirable since the elevated pressures within the vessel create tensile forces in the vessel walls. In contrast, solid wall vessels are normally heat treated to relieve them of the stress concentrations resulting primarily from the welds that are made during fabrication.
Heretofore different procedures have been developed for fabricating cylindrical shells from multiple layers, one highly successful procedure being set forth in U.S. Pat. No. 4,478,784. Heads, by reason of their compound curvatures are not easily fabricated in multiple layers, and as a consequence most heads are still of the solid wall construction. Thus, to a large measure, the limitations of present pressure vessels are set by the heads at their ends.